A297646 Heptagonal numbers (A000566) in which parity of digits alternates.

1, 7, 18, 34, 81, 189, 616, 783, 874, 3010, 4141, 4347, 5452, 6943, 8323, 12145, 14707, 18361, 52345, 69472, 74563, 78943, 96727, 129618, 147258, 163456, 214183, 232105, 250747, 258727, 270109, 276723, 278389, 307476, 309232, 381616, 389470, 436183, 450925

Offset: 1

Author

Colin Barker, Jan 02 2018

Keywords

• nonn
• base

Comments

Intersection of A000566 and A030141. - Felix Fröhlich, Jan 03 2018

Examples

			6943 is in the sequence because 6, 9, 4 and 3 have even and odd parity alternately.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..500

Crossrefs

Cf. A297644, A297645, A297647.

Cf. A000566, A030141, A030152, A030160, A068882.

Programs

Mathematica

Join[{1,7},Select[PolygonalNumber[7,Range[1000]],Union[Abs[Differences[ Boole[ OddQ[ IntegerDigits[ #]]]]]] =={1}&]] (* Harvey P. Dale, Jul 14 2022 *)

PARI

is_alt(n) = m=n; e=n%10; n\=10; while(n>0, f=n%10; if(e%2==f%2, return, e=f; n\=10)); return(m)
select(is_alt, vector(1000, n, (5*n^2-3*n)/2))